An on-surface microfluidic device controls fluid movement along a hydrophilic channel coated on a superhydrophobic surface. The device is expected as an alternative to overcome limits of a conventional microfluidic system based on a three-dimensional closed channel, for example, limits such that polydimethylsiloxane melts in an organic solvent and such a device is difficult to prepare or control since various factors such as a pump, valve and so forth should be satisfied. Mano et al., (2010) and Sagues et al. (2010) introduced a hydrophilic channel on a superhydrophobic surface through selective plasma treatment and silver deposition, and suggested a technique of flowing water through the hydrophilic channel. However, the hydrophilically functionalized part formed by the plasma treatment is not permanent; the hydrophilic properties are disappeared after a certain period of time. Thus, to achieve rapid mixing, modulation of a reaction time and scaling down of a reaction, a more stable and novel on-surface microfluidic technique capable of controlling water droplet-based microfluid is needed.